Question: The decay of uranium 238 to lead 206 is also used to

The decay of uranium-238 to lead-206 is also used to estimate the age of objects. Specifically, 206Pb-to-238U ratios allow dating of rocks. Why is the 238U decay to 206Pb useful for dating rocks but useless for daring objects 10,000 years old or younger? Similarly, why is carbon-14 dating useful for dating objects 10,000 years old or younger but useless for dating rocks?